Energy Transfers in CircuitsEdexcel GCSE Physics: Revision notes

When there is a current in a resistor, an energy transfer occurs that heats the resistor.

• Electrical energy is dissipated as thermal energy in the surroundings whenever a current does work against electrical resistance.
• This happens because moving electrons collide with the ions in the resistor's lattice structure, transferring energy to them and increasing their vibration — this raises the temperature of the resistor.
• Using low resistance wires reduces this unwanted energy transfer, which is why cables are often made of low-resistance metals like copper.

The heating effect of current has both advantages and disadvantages:

• Advantages: used deliberately in devices such as kettles, toasters, electric heaters and filament lamps, where the thermal (or light) energy produced is the useful output.
• Disadvantages: in devices such as motors, computers or transmission cables, heating is unwanted — it wastes energy and can cause components to overheat or be damaged, or wastes energy in transmission.

Energy transferred equation: E = I × V × t (energy transferred in joules = current in amperes × potential difference in volts × time in seconds).

• Power is the energy transferred per second, measured in watts (W).
• Power equation: P = E ÷ t (power = energy transferred ÷ time).
• The power transfer in any circuit device depends on both the potential difference across it and the current through it.
• Power equations: P = I × V and P = I² × R (both derived from combining V = I × R with P = I × V).

• The power rating of a domestic appliance (given in watts, e.g. on a kettle or hairdryer) tells you the rate at which it transfers energy.
• A higher power rating means the appliance transfers energy — and therefore changes stored energy (e.g. heats water, produces light, does mechanical work) — more quickly for the same time of use.
• Two appliances with different power ratings running for the same time will transfer different amounts of energy: the higher-power appliance transfers more energy in that time.